IV 



PHYLUM CCELENTERATA 



105 



than the radius of the umbrella, and by having a trichotomous 

 inter-radial canal and two unbranched adradial canals springing 

 from each gastric pouch. 



ORDER 1. STAUROMEDUS.E. 



The simplest member of this order, and indeed of the whole class of Scyphozoa, 

 is Tessera (Fig. 117), a small medusa about 4 mm. in diameter. It is interesting 



i.r. 



FIG. 117. Tessera princeps. A, external view; B, vertical section ; g.f. gastric filament; 

 gon. gouad ; -i.r. t. inter-radial tentacle; mnb. manubrium ; mth. mouth; p.r. t. per-radial 

 tentacle ; st. stomach ; tn. tsemole. (After Haeckel.) 



as having the same general characters as the Scyphula-stage of Aurelia, except 

 that the bell-shaped body is free-swimming. The edge of the umbrella is 

 surrounded by eight tentacles, four per- (j).r. t) and four inter-radial (i.r. t.), and 

 movement is effected by a well-developed system of circular and radial muscles. 

 The simplicity of the genus is well shown in the total absence of sense-organs. 

 The manubrium (mnb.) leads into a spacious stomach (tit.), from which four wide 

 per-radial pouches are continued into the umbrella, and are connected with one 

 another by a spacious cavity passing round its margin and called the circular 

 sinus. There are only four gastric filaments (g.f.), one springing from each of the 

 four inter-radial gastric ridges or tsenioles (tn. ). The gonads (gon. ) are horseshoe- 

 shaped, with their concavities directed towards the margin of the umbrella. 



